A. Field of the Invention
This invention relates to the field of art of band reject microwave filters.
B. Prior Art
Microwave band rejection filters have been generally defined as combinations of resonant and antiresonant circuits connected to transmission lines or waveguides so that an undesired band of frequencies is selectively attenuated from the total frequency spectrum. In the case of waveguide filters, the resonant circuits take the physical form of resonant cavities which are coupled to the guide by means of an iris in the wall of the guide. The specific cross-sectional shape of the iris (round, square, rectangular, etc.) permits certain propagation modes of R-F energy to pass from the guide into the cavity thereby providing the excitation energy sufficient to propagate various modes within the cavity. The irises associated with a given cavity are situated in odd multiple of .lambda..sub.g/4 where .lambda..sub.g refers to the main guide wavelength. Quarter wavelength iris spacing results in an energy loss or attenuation over a desired spectral range due to properly phased wave cancellation caused by the resonant behavior of the cavities.
The amount of attenuation of waveguide energy at any given frequency is determined by the shape and positioning of the coupling iris, the dimension and form of the cavity and the number of cavities employed in a given band rejection filter configuration. Attenuation characteristics may be predicted by utilizing lumped element prototype filter models which are well known in the art of conventional network synthesis techniques.
Coupled propagation modes may coexist at certain points within the cavity volume and have a real Poynting's vector existing at such points for a given pair of modes. Power contained in the spectral lines will be transferred between such modes on a known basis at a given point within the cavity. Such coupling is the equivalent behavior of an iris located between two single mode cavities thereby allowing energy flow between the two cavities. Uncoupled modes have no transfer of power from one mode to another mode within the cavity. In the uncoupled case, the electric and magnetic fields are orthogonal at all points within the cavity. In the coupled case, the fields are forced to be nonorthogonal at a known point on the cavity wall.
Band reject waveguide filters that provide microwave band reject filtering have in the past used rectangular or cylindrical cavities spatially situated along a straight section of waveguide in which each cavity has supported a single mode of propagation. The irises associated with each such cavity have been located at a distance of (2N-1).lambda..sub.g/4 apart, along the length of a straight waveguide section. However, such configurations in the prior art have had the disadvantage of large physical size, physical complexity and high fabrication costs.
Accordingly, an object of the present invention is to introduce a new class of band reject filters which utilize dual mode propagation behavior as the principal filtering technique and providing simplicity and compactness.